Oftentimes, hardwood lumber drying begins with exposure to an uncontrolled outside environment in a process called air drying. At times, loss of quality in this process can be in excess of 10 percent. Yet, with well managed air drying, including the use of plastic mesh and open sheds, quality losses can be under 2 percent. Almost all defects in lumber drying occur at very high moisture contents (MCs). After air drying, the lumber is put into a kiln to achieve the low final MCs desired. Quality losses in air drying cannot be repaired in the kiln-drying process.
Kiln drying of grade hardwood lumber is carried out in a closed chamber or building in which heated, humidity-controlled air is rapidly circulated over the surface of the wood being dried. Defects that might develop in drying are minimized by controlling the temperature, RH and velocity.
Temperatures. Conventional dry kilns commonly use initial drying temperatures, when the lumber is more than 50 percent MC, from 100 to 130 degrees F. As the lumber dries, temperatures are gradually raised. When the lumber is under 15 percent MC, temperatures, depending on species, range from 150 to 200 degrees F; although 160 degrees F maximum is preferred in most cases.
Air Flow. Air velocities through the load in drying hardwoods generally are between 200 and 650 fpm; the lower velocity values (250 fpm maximum) are for refractory or difficult-to-dry species such as oak and beech when at higher MCs, while the higher numbers are for the white species like maple, ash, and basswood. Velocity is not a critical factor for well air-dried lumber.
Humidities. Control of RH or EMC during kiln drying is necessary to avoid creating shrinkage associated defects, such as cracks, as well as to equalize and condition the wood with a high degree of precision. Today, many kilns use computerized recorders and controls.
A kiln schedule, which indicates the desired temperature, humidity, and velocity in the dryer depending on the MC of the lumber, is a carefully worked-out compromise between the need to dry lumber as fast as possible and the need to avoid severe drying conditions that will cause drying defects. It is a series of dry- and wet-bulb temperatures that establish the temperature and RH in the kiln and are applied at various stages of the drying process. Temperatures are chosen to strike this compromise of a satisfactory drying rate and avoidance of objectionable drying defects. One of the key defects is discoloration; another is checking and cracking.
As lumber is dried using kiln schedules, which are combinations of temperature and RH applied at various MC levels during drying, some means of estimating MC of the lumber, especially the wettest and driest pieces of lumber, in the kiln during drying is necessary. These MC values are determined by using 12 carefully chosen kiln samples. The samples chosen must represent the lumber and its variability. The main principle of selection is that the kiln samples be representative of the wettest and driest lumber in the kiln. (A large kiln might have 10,000 pieces of lumber. So "to put all your eggs in one basket" by using only 12 pieces of lumber to determine what is happening to all 10,000 pieces is risky. Poor sampling means low lumber quality when the customer tries to process the lumber.
The samples are spread throughout the kiln at various heights and distances from the ends of lumber stacks and that the samples are subject to the same airflow as the lumber.
Traditionally, kiln samples have been removed from the kiln periodically and weighed manually for MC estimates. This manual procedure is still used in the majority of hardwood operations, but automated methods are available.
The selection, preparation, placement, and weighing of kiln samples, if properly done, provides information that enables a kiln operator to 1) reduce drying defects, 2) control of the final MC of the charge better, 3) reduce drying time while maintaining lumber quality, 4) develop time schedules, and 5) locate kiln performance problems. All these advantages add up to lower drying costs and lower secondary manufacturing costs due to dried lumber that has a consistent MC, is uniformly conditioned, and free of drying defects.
After the lumber achieves the correct moisture content, it must be equalized and conditioned. That is, equalizing and conditioning (also called stress relief) are two quality-control measures necessary to complete the drying of high-quality hardwoods. Having the lumber at the desired final MC with little variability and free of drying stresses is critical in today's manufacturing and supplier/purchaser environment.
Frequently, near the end of the drying run, the MC of lumber varies considerably. This is because of natural variability in drying rate (such as the ends of the lumber versus the center of the lumber), initial MC, heartwood and sapwood and bacterial wet pockets. Variability can also result due to variability in drying conditions (temperature, humidity, or velocity) in various parts of the kiln. Variation in final MC can cause serious problems in the subsequent processing and use of the lumber. The purpose of equalizing is to reduce this variation in MC without over-drying.
Residual drying stresses (often called casehardening although there is no actual hardening of the surface) can cause problems with immediate warp when machining, various gluing problems, and pinching the saw blade when ripping or resawing. Drying stresses should be removed (that is, the lumber should be conditioned) in most hardwood lumber before the lumber is cut-up. The purpose of conditioning is to relieve the residual compressive drying stresses in the shell by exposure to high temperature and high RH. Conditioning can also have the beneficial effect of producing more uniform MC throughout the thickness of the boards. Effective equalizing is necessary before satisfactory conditioning can be accomplished because the effectiveness and length of the conditioning treatment depends on MC.
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